Power toothbrushes are in general well known and encompass a wide variety of designs and physical arrangements. Some power toothbrushes have proven to be both effective and commercially successful. One example is the toothbrush shown in U.S. Pat. No. 5,189,751, which is owned by the assignee of the present invention.
Many power toothbrushes have a rotary-type motion. Some have the capability of a 360° armature rotation, but due to design arrangements produce an oscillatory movement limited to a particular range of motion, i.e. a selected arcuate portion of 360°, in order to provide a more suitable brushing effect. Some of these rotary motion devices are mechanically driven, while others are resonant systems, involving a movable mass such as a brushhead structure and a spring which is attached to the handle. The resonant frequency of the system is affected by the handle.
The present invention is concerned with resonant systems. Resonant devices, using a motor with a rotating armature, often have the advantages of design simplicity and reasonable cost, as well as generally a small size, but also have significant disadvantages, including vibration coupled to the handle of the device caused by the reversing action of the system as it oscillates back and forth through the selected arc, instead of rotating continuously through a 360° rotation. The vibration is coupled to the handle through the spring element. The resonant frequency of such a system is affected by the moment of inertia (MOI) of the handle, as well as the other parts of the drive system. Performance of the device is also affected by the dynamics of the handle, such as damping, spring rate and moment of inertia. Further, when a user grips the handle of such a simple resonant system device, the user becomes part of the system, which further changes the dynamics of the handle and the resulting performance of the device. Thus, in a resonant system, many variables affect the dynamic characteristics of the handle and system performance. In addition, the spring coupling to the handle in a resonant system produces a significant physical vibration in the handle. These disadvantages have affected the commercial success of such toothbrushes.
It is thus desirable to significantly reduce the physical vibration in the handle and the noise produced by such a resonant drive system as well as increasing the efficiency of such a system.